The invention is based on a priority application EP 02 360 332.7 which is hereby incorporated by reference.
The invention relates to a polarization mode dispersion (PMD) controller device for controlling the state of polarization of an optical light wave, in particular in a terabit optical network, comprising a dispersion compensation unit and an adaptation control unit, wherein the dispersion compensation unit is fed with an incoming optical light wave, wherein the dispersion compensation unit comprises a multitude of compensation stages processing the optical light wave, wherein the dispersion compensation unit provides an equalized optical light wave, and wherein the adaptation control unit controls the dispersion compensation unit.
A device of this kind has been disclosed in F. Buchali et al., OFC 2002, Anaheim (Calif.), Proc. WE6.
For transporting information, in particular digital information, in large amounts and over long distances, optical information networks can be applied. The information to be transported is modulated onto optical signals such as light waves, and the modulated light waves are sent through an optical waveguide. The length of such a waveguide may be on the order of tens or even hundreds of kilometers.
When travelling through an optical waveguide, such as a glass fiber, the light wave changes its properties. In particular, the polarization of the optical light wave drifts during propagation, in particular as a function of frequency. The degree of polarization drift is varying over time, e.g. because of a changing temperature of the waveguide. This might lead to changing temporal broadening of the modulated bit pulses which results in a degradation of the detected signal, i.e. in a degraded bit error ratio. In order to recover the information carried by the optical light wave, the light wave is processed in a polarization mode dispersion (PMD) controller device which compensates for the polarization drift. Only after this processing, the equalized optical light wave can be fed into a detector device for recovering the information.
PMD controller devices of the state of the art comprise a dispersion compensation unit, a feedback signal tap and an adaptation control unit. The incoming optical light wave is fed into the dispersion compensation unit, is processed applying a multitude of tunable compensation units, typically comprising phase shifters, and exits the dispersion compensation unit as an altered optical light wave. The altered signal is tapped with the feedback signal tap and analyzed in the adaptation control unit (or in a separate feedback signal analyzer connected to the adaptation control unit). The adaptation control unit controls the tunable phase shifters. In order to optimize the altered signal, the control parameter of each phase shifter is dithered, and depending on the change of the quality of the feedback signal, the control parameter is adapted correspondingly.